REVISED 3/21/2009 - Newborn screening for Pompe disease is available and enzyme replacement therapy (ERT) with human acid alpha glucosidase (hGAA) is effective, ifERT is started early and immune tolerance is present. Knowledge of the mechanisms for immune tolerance to hGAA in Pompe disease will guide the design of future clinical trials for Pompe disease and other lysosomal storage disorders.Wepropose to use an innovative strategy to express hGAA in the liver that simultaneously leads to immune tolerance to hGAA and long lasting efficacy from ERT in Pompe disease mice. Using these model systems, we will investigate the immunologic mechanisms required to induce immune tolerance in GAA-null mice, and evaluate the impact of immune tolerance on efficacy of ERT or gene therapy. This strategy can be translated to clinical applications very readily, because unlike other gene therapy approaches the vector dose is very low, risks are similarly low, and potential benefits are great.General Hypothesis: Liver-specific expression of hGAA will induce immune tolerance and enhance the efficacy of replacement therapy (ERT and/or gene therapy) in Pompe disease. Specific Aim 1 will evaluate mechanisms responsible for the induction of immune tolerance to hGAA by Pompe disease. Liver-specific expression of hGAA with a tolerogenic AAV vector will establish immune tolerance in GAA-null mice. Immune tolerance to hGAA will be demonstrated via challenges with recombinant hGAA and adjuvant. The role of key players of the adaptive immune response, including Treg cells and NKT cells, will be examined in the presence or absence of immune tolerance. Treg cell depletion will prevent immune tolerance to hGAA. Specific Aim 2 will demonstrate that efficacy of the hGAA therapy can be improved by modulating the immune response to hGAA. A dose reduction experiment with tolerogenic vector will determine a lower threshold for achieving immune tolerance to hGAA. Long-term efficacy from ERT will be enhanced by immune tolerance, because adaptive immune responses will be abrogated by the less immunogenic transgene. The benefit of immune tolerance to hGAA will be demonstrated by evaluating ERT in GAA-null mice that received low-dose tolerogenic vector. We will evaluate an innovative approach to immunodulatory gene therapy, consisting of desensitization to the hGAA protein by the administration of low-dose tolerogenic vector to mice that previously formed antibodies to hGAA. Desensitization prevents mortality from anaphylaxis during ERT in antibody-positive GAA-null mice. These comparisons will guide preclinical experiments to further immunomodulatory gene therapy in Pompe disease and other lysosomal storage disorders.